A. Men. According to new research from the University of Barcelona, men feel more alert—more quickly—than women do after drinking a caffeinated beverage. In the study, participants sipped a caffeinated espresso, then rated how alert they felt 10, 20 and 30 minutes later. Men reported feeling less drowsy after only 10 minutes and sustained the mental boost for the entire half hour. Women got a "kick" from the espresso, too, but rated it weaker than the men did.

And as it turns out, just anticipating the stimulating effects of caffeine may be enough—particularly for women—to feel them. Using the same methods, researchers also looked at the mental effects of decaffeinated coffee. Ten minutes after drinking a cup of decaf, women reported feeling significantly more alert. The men reported only a slight mental boost. "Women may be more likely than men to experience a placebo response [to caffeine]," says Ana Adan, Ph.D., lead author and research head of the Psychopharmacology and Drug Dependence Group at the University of Barcelona.